One well-known gas phase hydrogenation reaction is the hydrogenation of nitrogen to form ammonia. This reaction is carried out on a large commercial scale by the Haber-Bosch process, or a modification, in which a gaseous mixture of hydrogen and nitrogen is passed in contact with a solid catalyst whereupon a portion of the hydrogen and nitrogen in the mixture reacts to form ammonia. The ammonia is removed from the resulting gas mixture by any of several techniques, and the unreacted hydrogen and nitrogen pass to a further catalytic reaction stage or are recycled to the same reaction stage.
It has long been recognized that the equilibrium yield for the reaction is high at low temperature and decreases as temperature increases. However, the rate of reaction at low temperature is very low, and even in the presence of the known catalysts for the reaction the temperature of the reaction must be high in order to effect a satisfactory rate of conversion. In order to compensate for the low equilibrium yield at the higher temperature, current commercial processes are operated at high pressure. Typically commercial ammonia production is carried out in the reactor generally in the range 350.degree. C. to 560.degree. C. at 100 atmospheres to 1000 atmospheres in the presence of a promoted iron catalyst. Conversion to ammonia from the stoichiometric mixture during one pass of the mixture through the reactor is generally in the range 15% to 25%. Typical equilibrium yields with stoichiometric quantities of hydrogen and nitrogen and no impurities are shown in the following table:
______________________________________ Pressure, psi Temp. .degree.C. Equilibrium Yield, % ______________________________________ 500 300 9 500 200 50 1500 200 77 3000 300 52 ______________________________________
It is apparent from these figures that if a catalyst which is active at lower temperatures can be developed, then very high yields can be obtained at low temperature. If the reaction pressure is maintained high, even high equilibrium yields are available and this would result in a significant advantage in terms of increased ammonia production rates. Alternatively, the pressure can be reduced to reduce energy costs while still maintaining a good production rate.
For many years the catalysts employed for the commercial production of ammonia have been promoted iron catalysts because these materials have been found to be both active and stable. Promoted iron is prepared by melting iron oxide, together with promoter components such as Al.sub.2 O.sub.3, CaO, BaO, K.sub.2 O, ZrO.sub.2, or SiO.sub.2, cooling and crushing the resulting oxide, and then reducing the oxide with hydrogen to produce a granular porous iron structure containing the promoters in dispersed form. The material must be cleaned or conditioned before use, usually by heating it in the presence of hydrogen. A typical catalyst for ammonia production is a triply promoted iron containing 90 weight % iron and 10 weight % Al.sub.2 O.sub.3, CaO and K.sub.2 O having a surface area of 3 to 12 square meters per gram.